Pyrolysis processes, such as steam cracking, are utilized for converting saturated hydrocarbons to higher-value products such as light olefins, e.g., ethylene and propylene. Besides these useful products, hydrocarbon pyrolysis can also produce a significant amount of relatively low-value heavy products, such as pyrolysis tar. When the pyrolysis is conducted by steam cracking, the pyrolysis tar is identified as steam-cracker tar (“SCT”).
Pyrolysis tar is a high-boiling, viscous, reactive material comprising complex, ringed and branched molecules that can polymerize and foul equipment. Pyrolysis tar also contains high molecular weight non-volatile components including paraffin insoluble compounds, such as pentane insoluble compounds and heptane-insoluble compounds. Particularly challenging pyrolysis tars contain >1 wt % toluene insoluble compounds. The high molecular weight compounds are typically multi-ring structures that are also referred to as tar heavies (“TH”). These high molecular weight molecules can be generated during the pyrolysis process, and their high molecular weight leads to high viscosity which limits desirable pyrolysis tar disposition options. For example, it is desirable to find higher-value uses for SCT, such as for fluxing with heavy hydrocarbons, especially heavy hydrocarbons of relatively high viscosity. It is also desirable to be able to blend SCT with one or more heavy oils, examples of which include bunker fuel, burner oil, heavy fuel oil (e.g., No. 5 or No. 6 fuel oil), high-sulfur fuel oil, low-sulfur oil, regular-sulfur fuel oil (“RSFO”), Emission Controlled Area (ECA) fuel with <0.1 wt % sulfur and the like.
One difficulty encountered when blending heavy hydrocarbons is fouling that results from precipitation of high molecular weight molecules, such as asphaltenes. See, e.g., U.S. Pat. No. 5,871,634, which is incorporated herein by reference in its entirety. In order to mitigate asphaltene precipitation, an Insolubility Number, IN, and a Solvent Blend Number, SBN, are determined for each blend component. Successful blending is accomplished with little or substantially no precipitation by combining the components in order of decreasing SBN, so that the SBN of the blend is greater than the IN of any component of the blend. Pyrolysis tars generally have high SBN>135 and high IN>80 making them difficult to blend with other heavy hydrocarbons. Pyrolysis tars having IN>100, e.g., >110, e.g., >130, are particularly difficult to blend without phase separation occurring.
Attempts at pyrolysis tar hydroprocessing to reduce viscosity and improve both IN and SBN have not led to a commercializable process, primarily because fouling of process equipment could not be substantially mitigated. For example, neat SCT hydroprocessing results in rapid catalyst coking when the hydroprocessing is carried out at a temperature in the range of about 250° C. to 380° C., a pressure in the range of about 5400 kPa to 20,500 kPa, using a conventional hydroprocessing catalyst containing one or more of Co, Ni, or Mo. This coking has been attributed to the presence of TH in the SCT that leads to the formation of undesirable deposits (e.g., coke deposits) on the hydroprocessing catalyst and the reactor internals. As the amount of these deposits increases, the yield of the desired upgraded pyrolysis tar (upgraded SCT) decreases and the yield of undesirable byproducts increases. The hydroprocessing reactor pressure drop also increases, often to a point where the reactor is inoperable.
One approach taken to overcome these difficulties is disclosed in International Patent Application Publication No. WO 2013/033580, which is incorporated herein by reference in its entirety. The application discloses hydroprocessing SCT in the presence of a utility fluid comprising a significant amount of single and multi-ring aromatics to form an upgraded pyrolysis tar product. The upgraded pyrolysis tar product generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the SCT feedstock, resulting in improved compatibility with fuel oil and blend-stocks. Additionally, efficiency advances involving recycling a portion of the upgraded pyrolysis tar product as utility fluid are described in International Patent Application Publication No. WO 2013/033590 also incorporated herein by reference in its entirety.
U.S. Published Patent Application No. 2015/0315496, which is incorporated herein by reference in its entirety, describes separating and recycling a mid-cut utility fluid from the upgraded pyrolysis tar product. The utility fluid comprises ≥10.0 wt % aromatic and non-aromatic ring compounds and each of the following: (a) ≥1.0 wt % of 1.0 ring class compounds; (b) ≥5.0 wt % of 1.5 ring class compounds; (c) ≥5.0 wt % of 2.0 ring class compounds; and (d) ≥0.1 wt % of 5.0 ring class compounds.
U.S. Published Patent Application No. 2015/0368570, which is incorporated herein by reference in its entirety, describes separating and recycling a utility fluid from the upgraded pyrolysis tar product. The utility fluid contains 1-ring and/or 2-ring aromatics and has a final boiling point ≤430° C.
U.S. Published Patent Application No. 2016/0122667, which is incorporated herein by reference in its entirety, describes a process for upgrading pyrolysis tar, such as steam cracker tar, in the presence of a utility fluid which contains 2-ring and/or 3-ring aromatics and has solubility blending number (SBN)≥120.
Despite these advances, there remains a need for further improvements in the hydroprocessing of pyrolysis tars, especially those having high IN values, which allow the production of upgraded tar product having lower viscosity and density specifications without compromising the lifetime of the hydroprocessing reactor.